Clarify cache entry removal in ReferenceManager

Add comment to clarify cache entry removal process.

Author: memurats

lib/private/Collaboration/Reference/ReferenceManager.php

@@ -172,6 +172,7 @@ public function invalidateCache(string $cachePrefix, ?string $cacheKey = null):
 			return;
 		}
 
+		// remove specific cache entry; using ($cacheKey ?? '') avoids md5(null) warnings and ensures stable hashing
 		$this->cache->remove(md5($cachePrefix) . '-' . md5($cacheKey ?? ''));
 	}